This invention relates to wavelength or frequency shifting and amplification of optical radiation and more particularly to the use of new polarization dependent four-wave-mixing processes to generate new wavelengths of optical radiation. The invention further relates to the use of birefringent single mode optical waveguide fibers for the tunable generation of new wavelengths of optical radiation.
It is very desirable to be able to produce coherent laser-like radiation over a broad spectrum of wavelengths or frequencies. This is based on the fact that a given frequency is often better suited for a particular application than another frequency.
Applications such as interferometry, plasma fusion research, target acquisition or tracking, communications through optical fibers, or medical applications all exhibit wavelength preferences. Each of these applications has an ideal wavelength for which the laws of physics and optics dictate the most efficient transfer functions. In laser fusion work, for example, fuel targets have complex structures which are best penetrated, vaporized, and imploded by particular wavelengths of radiation, depending on their atomic composition. In communications, certain frequencies of optical radiation traverse optical fiber waveguides with lower losses than others. In medicine, it is known that some types of tissue and biological structures absorb particular wavelengths of radiation more readily than others.
Therefore, there have been efforts to provide various sources of coherent radiation which are tunable over a broad range. One approach is the use of dye lasers wherein an amplifying medium consisting of a specialized dye is optically pumped, by a laser source, to produce stimulated emission at a new wavelength. By changing the composition of the dye different output wavelengths are achieved, thus allowing the output wavelength to be tuned.
However, there are at least three drawbacks. Firstly, the frequency control for dye lasers is, as yet, not fully developed and often no dyes are available to reach some wavelengths for a given pump laser. This is due to a relatively large minimum wavelength shift in the dye laser output. Therefore, new wavelengths very close to given input wavelengths cannot be reached. Secondly, dyes have limited lifetimes and require constant replenishment. Thirdly, and perhaps most important, dye lasers are often relatively complex and cumbersome compared to the applications desired.
Another approach is the use of various scattering processes such as Stimulated Raman Scattering (SRS) or Stimulated Brillouin Scattering (SBS). Here gaseous media such as benzene, ammonia, parahydrogen or carbon monoxide interact with photons through molecular or atomic scattering to alter their energy. This produces backward or forward scattered light at new wavelengths. This method typically produces smaller frequency shifts than the dye laser technique. However, the frequency shifts are on the order of 1000 cm.sup.-1 or more, leaving some wavelengths unreached. Also, Raman cells and SBS media support multiple modes of resonant light, unless special filters or optics are utilized, which are not useful for many optical waveguide applications.
More recently, photon mixing and dispersion processes in optical fibers have been used to produce new wavelengths of optical radiation. This is especially useful in the area of wavelength or frequency shifters, decoders, multiplexers, or interferometers as used in optical fiber communication or processing systems.
One method is illustrated in U.S. Pat. No. 3,875,422 issued to R. H. Stolen, wherein a multimode optical fiber waveguide is used. A pumping beam is launched into two distinct modes of the fiber while a third "signal" beam is also launched into the fiber. The overlapping modes or beams interact in the fiber and through parametric interaction yield an output at a new wavelength. While useful, this method uses multimode fibers in which the interaction is inefficient because of limited spatial overlap of the modes. Also multi-mode fibers are not easily useable with single mode waveguide systems because of the differences in the spatial profiles of the modes.
Another method utilizing single mode fibers is illustrated in U.S. Pat. No. 4,255,017 issued to A. Hasegawa. In Hasegawa, anomalous dispersion of a single mode optical fiber is used to create a modulation instability. The instability creates new sideband waves from an input wave which interact with the input wave in a form of photon mixing. The result is a new wave of new wavelength.
These two methods are an improvement over dye lasers but they do not achieve wavelength shifts much below 1000 cm.sup.-1. A third technique using optical fibers has been demonstrated by R. H. Stolen et al and is noted in "Phase Matching in Birefringent Fibers," R. H. Stolen, M. A. Bosch, and Chinlon Lin, OPTICS LETTERS, 1981, Vol. 6, No. 5, pp. 213-215.
Stolen et al use a birefringent fiber to establish a differential velocity for a pump wave versus Stokes and anti-Stokes waves produced in an optical fiber waveguide. Stolen et al transmit the pump waves in what is termed the slow axis of a birefringent fiber, to generate a Stokes wave polarized along the fast axis and an anti-Stokes wave also polarized along the fast axis. The net effect is to improve the phase matching of these waves so as to produce parametric interactions (amplification) between them and yield outputs at the Stokes and anti-Stokes wavelengths. This method allows the production of shifts on the order of 500 cm.sup.-1 in short fibers for efficiencies on the order of 10%. Stolen et al discovered that the Stokes and anti-Stokes waves disappear when the pump waves are launched into the fast axis of the optical fiber.
While this is a further improvement in the art, there is still a desire to achieve smaller wavelength shifts &lt;200 cm.sup.-1. In addition, higher efficiencies at given powers are needed to provide commercial viability for such wavelength converters.